Method for the production of propylene oxide

ABSTRACT

A process for the preparation of propylene oxide in which 
     (i) propene is reacted with hydrogen peroxide in the presence of methanol to give propylene oxide, giving a mixture (Gi) comprising propylene oxide, methanol, water and unreacted hydrogen peroxide, 
     (ii) a mixture (Gii) comprising methanol, water and hydrogen peroxide is separated off from the mixture (Gi), giving a mixture (Ga) comprising propylene oxide, and 
     (iii) water is separated off from the mixture (Gii), giving a mixture (Giii) comprising methanol and methyl formate.

This application is a 371 of PCT/EP01/07716 filed Jul. 5, 2001.

The present invention relates to a process for the preparation ofpropylene oxide from propene and hydrogen peroxide in the presence ofmethanol. After the reaction of the propene with hydrogen peroxide inthe process according to the invention, a mixture comprising methanol,water and unreacted hydrogen peroxide is separated off from the reactiondischarge, and this mixture is subjected to a separation process whichgives a further mixture comprising methanol and methyl formate.

Processes for the preparation of propylene oxide from propene are knownfrom the prior art. In these processes, the problem generally occursthat a certain amount of hydrogen peroxide is not reacted during thereaction and arises on subsequent removal of propylene oxide from thereaction discharge.

In order to remedy this problem, it has been proposed, inter alia, toseparate off hydrogen peroxide, in an intermediate separation step, fromthe reaction discharge from a first reaction step and to react it againwith alkene in a second reaction step. Such processes are described, forexample, in PCT/EP99/05740 and DE-A 100 15 246.5. Although it ispossible here to achieve virtually one hundred per cent hydrogenperoxide conversion in the second reaction step, the second reactionstep does mean, however, that increased complexity is necessary.

It is an object of the present invention to provide a process in whichthe problem of the unreacted hydrogen peroxide arising is solvedinexpensively and efficiently.

We have found that this object is achieved by a process for thepreparation of propylene oxide in which

(i) propene is reacted with hydrogen peroxide in the presence ofmethanol to give propylene oxide, giving a mixture (Gi) comprisingpropylene oxide, methanol, water and unreacted hydrogen peroxide,

(ii) a mixture (Gii) comprising methanol, water and hydrogen peroxide isseparated off from the mixture (Gi), giving a mixture (Ga) comprisingpropylene oxide, and

(iii) water is separated off from the mixture (Gii), giving a mixture(Giii) comprising methanol and methyl formate.

Methanol is particularly preferably employed as solvent. It is alsopossible here to employ one or more further solvents in addition tomethanol. In principle, all solvents which are suitable for therespective reaction can be employed as such further solvents. Interalia, preference is given, for example, to

water,

alcohols, preferably lower alcohols, further preferably alcohols havingless than 6 carbon atoms, for example ethanol, propanols, butanols andpentanols,

diols or polyols, preferably those having less than 6 carbon atoms,

ethers, for example diethyl ether, tetrahydrofuran, dioxane,1,2-diethoxyethane and 2-methoxyethanol,

esters, for example methyl acetate or butyrolactone,

amides, for example dimethylformamide, dimethylacetamide andN-methylpyrrolidone,

ketones, for example acetone,

nitrites, for example acetonitrile,

or mixtures of two or more of the above-mentioned compounds.

In the process according to the invention, the reaction of propene withhydrogen peroxide is preferably carried out in the presence of acatalyst. Feasible catalysts for the conversion of propylene intopropylene oxide are in principle all catalysts, preferably allheterogeneous catalysts, which are suitable for the respective reaction.

Preference is given to catalysts which comprise a porous oxidicmaterial, for example a zeolite. Preference is given to catalysts inwhich the porous oxidic material is a titanium-, vanadium-, chromium-,niobium- or zirconium-containing zeolite.

In particular, zeolites which contain no aluminum and in which some ofthe Si(IV) in the silicate lattice have been replaced by titanium asTi(IV) exist. The titanium zeolites, in particular those having acrystal structure of the MFI type, and possibilities for theirpreparation are described, for example, in EP-A 0 311 983 and EP-A 0 405978.

Titanium zeolites having an MFI structure are known for the fact thatthey can be identified via a certain pattern in the determination oftheir X-ray diffraction diagrams and in addition via a skeletalvibration band in the infrared region (IR) at about 960 cm⁻¹ and thusdiffer from alkali metal titanates or crystalline or amorphous TiO₂phases.

Suitable here are, in detail, titanium-, vanadium-, chromium-, niobium-and zirconium-containing zeolites having a pentasil zeolite structure,in particular the types with X-ray assignment to the ABW, ACO, AEI, AEL,AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC,APD, AST, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE,CAN, CAS, CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT,DOH, DON, EAB, EDI, EMT, EPI, ERI, ESV, EUO, FAU, FER, GIS, GME, GOO,HEU, IFR, ISV, ITE, JBW, KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN,MAZ, MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW,MWW, NAT, NES, NON, OFF, OSI, PAR, PAU, PHI, RHO, RON, RSN, RTE, RTH,RUT, SAO, SAT, SBE, SBS, SBT, SFF, SGT, SOD, STF, STI, STT, TER, THO,TON, TSC, VET, VFI, VNI, VSV, WEI, WEN, YUG or ZON structure and tomixed structures consisting of two or more of the above-mentionedstructures. Also feasible for use in the process according to theinvention are titanium-containing zeolites having the UTD-1, CIT-1 orCIT-5 structure. Further titanium-containing zeolites which may bementioned are those having the ZSM-48 or ZSM-12 structure.

Ti zeolites having the MFI, MEL or MFI/MEL mixed structure are regardedas particularly preferred for the process according to the invention.Preference is furthermore given, in detail, to the Ti-containing zeolitecatalysts generally known as “TS-1”, “TS-2” and “TS-3”, and Ti zeoliteshaving a skeletal structure which is isomorphous with beta-zeolites.

In the process according to the invention, particular preference isgiven to a heterogeneous catalyst comprising the titanium-containingsilicalite TS-1.

Accordingly, the present invention also relates to a process asdescribed above in which, for the preparation of the propylene oxide, azeolite catalyst, preferably a titanium silicalite catalyst and inparticular a titanium silicalite catalyst of the structure TS-1, isemployed.

The removal of water from the mixture (Gii) is in the process accordingto the invention preferably carried out by distillation, it beingpossible to use one or alternatively a plurality of distillationcolumns. Use of one or two distillation columns is preferred. In thecase that heat recovery is unnecessary, one distillation column ispreferably used. Two or more distillation columns are preferably used ifparticularly good heat integration in the process is to be ensured.

Regarding the physical parameters, such as temperature or pressure,there are no particular restrictions during removal of water from themixture (Gii) by distillation, so long as it is ensured that hydrogenperoxide is degraded during the distillation and a mixture (Giii)comprising methyl formate and methanol is obtained.

If only one column is employed for the removal of water from the mixture(Gii) in the process according to the invention, this preferably has atleast 5, preferably at least 20 and further preferably at least 30theoretical plates. The distillation is preferably carried out atpressures in the range from 0.5 to 40 bar, preferably from 1.0 to 20 barand particularly preferably from 2 to 15 bar.

If two columns are employed for the removal of water from the mixture(Gii) in the process according to the invention, the pressures are thenselected in such a way that the heat of condensation at the top of thecolumns can be used to heat other process streams. This is achieved, forexample, by cooling the condenser of at least one column using, forexample, water and employing the hot water resulting from the cooling orthe steam resulting from the cooling to heat one or more steps of theprocess according to the invention or even one or more other processes.

The first distillation column is preferably operated at pressures in therange from 0.5 to 40 bar and preferably from 1 to 20 bar. In a possibleembodiment, the first column is operated at a higher pressure level thanthe second column. In this case, the bottom of the second column isheated using the vapors from the first column. In a preferredembodiment, the first column is operated at a lower pressure level thanthe second column. In this case, the bottom of the first column isheated using, the vapors from the second column.

In a particularly preferred embodiment, the first column is operated atpressures in the range from 4 to 9 bar and further preferably in therange from 6 to 8 bar and the second column is operated at pressures inthe range from 11 to 16 bar and further preferably in the range from 12to 14 bar. In general, from 20 to 80%, preferably from 30 to 70% andparticularly preferably from 40 to 60% of the methanol present in themixture (Gii) is separated off at the top of the first column togetherwith methyl formate. The mixture obtained at the bottom of the firstcolumn is used as feed for the second column. The top product from thesecond column comprises the residual methanol and methyl formate, andthe bottom product comprises water. The top product from the firstcolumns and the top product from the second column are combined to givethe mixture (Giii).

Both in the case of removal of water in two columns and in the case ofremoval of water in one column, the separation conditions areparticularly preferably selected so that the water content in themixture (Giii) is generally less than 3% by weight, preferably less than1% by weight and particularly preferably less than 0.3% by weight. Theseparation conditions are further preferably selected so that themethanol content in the bottom take-off is less than 5% by weight,preferably less than 1% by weight and particularly preferably less than0.2% by weight.

As further components, the bottom take-off may in addition comprise,inter alia, methoxypropanols, propylene glycol, formic acid, dipropyleneglycol monomethyl ether and formaldehyde, for example.

Accordingly, the present invention also relates to a process asdescribed above in which the water is separated off by distillation in(iii), wherein

(w) a mixture (Gw) which principally comprises methanol and methylformate is separated off from the mixture (Gii) at the top of a firstdistillation column,

(x) the mixture obtained at the bottom of the first distillation columnis fed as feed to a second distillation column,

(y) a mixture (Gy) which principally comprises methanol and methylformate is obtained at the top of the second distillation column, and

(z) the mixtures (Gw) and (Gy) are combined to give the mixture (Giii).

The mixture (Giii) obtained after the removal of water is, in apreferred embodiment of the process according to the invention, fed to afurther work-up step, in which methanol is separated from methyl formatein the mixture (Giii).

In a further preferred embodiment of the process according to theinvention, the methanol obtained in this way is recycled into (i).

The present invention therefore also relates to a process as describedabove in which methanol is separated from methyl formate in the mixture(Giii), and the methanol separated off is recycled into (i).

The removal of the methanol from the mixture (Giii) comprising methylformate and methanol can in principle be achieved here by allconceivable methods, so long as it is ensured that the purity of theseparated off methanol satisfies the demands set.

Mention may be made here, inter alia, of chemical methods. For example,it is possible to bring the mixture comprising methanol and methylformate into contact with a suitable basic ion exchanger, producingmethanol, while the formate remains on the ion exchanger. This processis described, inter alia, in U.S. Pat. No. 5,107,002.

Furthermore, the mixture comprising methanol and methyl formate can betreated with a base, with the methyl formate being hydrolyzed. All basesby means of which the hydrolysis of the methyl formate can be achievedcan be used here. Preference is given to strong bases. Particularlypreferred bases for the purposes of the present invention are salts ofacids which are weaker acids than formic acid. Inter alia, preference isgiven here to, for example, alkali metal and alkaline earth metalhydroxides and alkali metal salts of alcohols or phenols. It is ofcourse also possible to use mixtures of two or more of these bases.

The removal of methanol from the mixture comprising methanol and methylformate can furthermore preferably be carried out using physicalmethods, for example distillation methods.

Of these, extractive distillation methods, for example, as known fromthe prior art and described, for example, in the above-mentioned U.S.Pat. No. 5,107,002, are possible.

However, preference is given to distillation methods, which require lesscomplex equipment than said extraction distillation methods.

Preference is given to a distillation method in which one or morecolumns, further preferably one column, is employed. If one column isemployed, this has at least five, preferably at least 10 and inparticular at least 20 theoretical plates.

The pressures preferably used are generally in the range from 0.2 to 50bar, preferably in the range from 1.5 to 30 bar and in particular in therange from 2.0 to 20 bar.

The head temperatures and bottom temperatures are clearly determined bythe selected pressure. In a particularly preferred embodiment, thiscolumn, which has approximately 20 theoretical plates, is operated atpressures in the range from 2.0 to 20 bar. The top product obtained is amixture comprising methyl formate and a small proportion of the methanolpresent in the feed. This mixture preferably has a methanol content ofless than 80% by weight, preferably less than 50% by weight andparticularly preferably less than 20% by weight.

It is furthermore conceivable for the mixture (Giii) comprising methanoland methyl formate to contain additional further components besidesmethyl formate. The term “components” here denotes both pure compoundsand also azeotropes which have a boiling point which is lower than theboiling point of methanol. As such components, mention may be made,inter alia, by way of example of acetaldehyde, 1,1-dimethoxyethane,propionaldehyde, 1,1-dimethoxypropane, acetone or 2,4dimethyl-1,3-dioxolane. These can likewise be separated off from themixture during the work-up.

It is thus possible to separate off these by-products from the mixtureby one or more suitable physical or chemical methods before separationof the methanol from methyl formate. It is likewise possible firstly toseparate off methanol from the mixture, which can give a mixturecomprising methanol and at least one impurity. In this case, the removalof methanol from the mixture can be followed by one or more separationsteps in which methanol is separated from the at least one impurity. Theremoval of methanol from the mixture can likewise result in a mixturecomprising methyl formate and one or more impurities. This too can, ifnecessary, be separated into its constituents by one or more suitablephysical or chemical methods. The constituents can then be separated orfed together as starting materials to one or more further processes orsent to heat recovery.

Depending on the chemical nature of the impurities, it is also possibleto separate off methanol from the mixture by separating both methylformate and the at least one impurity from methanol in a single processstep.

The distillation preferably employed in accordance with the inventionand described as above gives a methanol fraction which has a content ofmethyl formate of in general less than 500 ppm, preferably less than 100ppm and in particular preferably less than 20 ppm.

Depending on the demands made of the purity of the methanol fraction,residues of other components, for example acetaldehyde,1,1-dimethoxyethane, propionaldehyde, 1,1-dimethoxypropane, acetone or2,4-dimethyl-1,3-dioxolane, remaining in the methanol fraction after thedistillative work-up can be separated from the methanol by one or moresuitable measures, for example one or more further distillations.

In general, it is entirely sufficient if the concentration of eachindividual secondary component in the methanol is less than 1% by weightand the sum of all secondary components does not exceed 5% by weight.

The methanol separated off from the methyl formate in this way can bere-used, it being in principle conceivable to recycle the methanol intothe process for the preparation of propylene oxide or, if necessary, tofeed it to a different process in which methanol is required as solventor as starting material or in another function. It is of courseconceivable to divide the methanol stream resulting from the separationaccording to the invention into two or more streams and to feed eachstream to a different process.

In a particularly preferred embodiment of the process according to theinvention, the methanol separated from methyl formate and, if necessary,from one or more secondary components or impurities is recycled, asdescribed above, into the process for the preparation of propyleneoxide. The methanol is preferably, inter alia, pumped into a buffer tankand fed into the process therefrom.

As far as the preparation of propylene oxide in (i) is concerned, thisis very particularly preferably carried out in one step. The term “onestep” as used for the purposes of the present application denotesprocess procedures in which no removal of hydrogen peroxide takes place.For the purposes of the present invention, a one-step process thuscovers, inter alia, processes in which the starting materials arereacted with one another in a reactor, and the reaction discharge isprocessed further, and also, inter alia, processes in which

propene is reacted with hydrogen peroxide in a first reaction step togive a product stream,

the product stream is fed to at least one intermediate treatment, with afurther product stream being obtained from the intermediate treatment,and

the further product stream is fed to a further reaction step, in whichhydrogen peroxide is reacted with propene,

where the intermediate treatments are not removals of hydrogen peroxide.

Further reaction steps are of course also conceivable, where anintermediate treatment can, but need not, take place between tworeaction steps. A conceivable intermediate treatment is, inter alia, forexample the addition of a base to a product stream, it particularlypreferably being possible to employ basic compounds which influence thereaction of hydrogen peroxide with propene in the desired manner in theprocess according to the invention. If, for example, zeolites areemployed as heterogeneous catalysts, preference is given to basiccompounds which lower the acidity of these zeolites. Such bases aredescribed, for example, in DE-A 100 15 246.5, which is incorporated intothe present application in its full scope in this respect by way ofreference.

Concerning, for example, the arrangement and type of the reactorsemployed in (i), all suitable reactors are also conceivable here. Inparticular, use can be made, for example in one or more of theabove-mentioned reaction steps, of two or more reactors connected inparallel. In this respect, reference is made to DE-A 100 15 246.5, whichis incorporated into the present application in its full scope withrespect to the possible reactor arrangements by way of reference.

The present invention therefore relates to a process as described abovein which the reaction in (i) is carried out in one step.

Furthermore, the temperature and pressure of the reaction medium can bechanged during the process in the course of the preparation of propyleneoxide from propene and hydrogen peroxide. The pH and temperature of thereaction medium can likewise be changed. It is furthermore possibleadditionally to change the pressure under which the reaction takes placein addition to the pH and temperature of the reaction medium. In thisrespect, reference is made to DE-A 199 36 547.4, which is incorporatedinto the present application in its full scope in this respect by way ofreference.

The preparation of propylene oxide in (i) is particularly preferablycarried out in such a way that the conversion of hydrogen peroxide isgenerally in the range from 85 to 99.99%, preferably in the range from90 to 99.9% and particularly preferably in the range from 95 to 99.5%.

The present invention therefore also relates to a process as describedabove in which the hydrogen peroxide conversion in (i) is in the rangefrom 85 to 99.99%.

In accordance with the invention, a mixture (Gii) comprising methanol,water and hydrogen peroxide is separated off from the mixture (Gi),giving a mixture (Ga) comprising propylene oxide. This separation cangenerally be carried out by any suitable method. The separation ispreferably carried out by distillation.

Preference is given to a distillation method in which one or morecolumns are employed, further preferably one column is employed. If onecolumn is employed, this has at least 5, preferably at least 10 and inparticular at least 15 theoretical plates. The pressures preferably usedare generally in the range from 0.5 to 10 bar, preferably in the rangefrom 0.8 to 3 bar and in particular in the region of the ambientpressure.

The top and bottom temperatures are clearly determined by the selectedpressure. In a very particularly preferred embodiment, this column,which has about 15 theoretical plates, is operated at ambient pressure.The top product obtained at a head temperature in the region ofapproximately 35° C. is a mixture (Ga) comprising propylene oxide. Thebottom product obtained at a bottom temperature of approximately 68° C.is a mixture (Gii) comprising methanol, water and unreacted hydrogenperoxide.

The mixture (Gii) may, in addition to methanol, water and hydrogenperoxide, comprise one or more further compounds which can be, forexample, by-products of the reaction in (i) or compounds formed duringthe separation in (ii), or compounds introduced into (i) as impuritiesin the starting materials, or solvents employed, for example, inaddition to the solvent methanol, or compounds employed for thedistillative separation in (ii), or compounds added, for example, duringan intermediate treatment as described above. In the preparation ofpropylene oxide from propene in (i), the mixture (Gii) may comprise,inter alia, 1,1 dimethoxyethane, acetone, acetaldehyde,1,1-dimethoxypropane, propionaldehyde, methyl formate and/or 2,4dimethyl-1,3-dioxolane.

In addition to propylene oxide, the mixture (Ga) may likewise compriseone or more further compounds, which can again be, for example,by-products of the reaction in (i) or compounds formed in the separationin (ii), or compounds introduced into (i) as impurities in the startingmaterials, or methanol or solvents employed, for example, in addition tothe solvent methanol, or compounds employed for the distillativeseparation in (ii), or compounds added, for example, during anintermediate treatment as described above.

Depending on the process procedure, it is conceivable in the processaccording to the invention for the mixture (Ga) obtained from thereaction discharge in (i) to comprise methanol in addition to propyleneoxide. Preferred methanol contents are in the range from 10 to 90% byweight, particularly preferably in the range from 25 to 75% by weightand very particularly preferably in the range from 40 to 60% by weightof methanol.

In a further preferred embodiment, inter alia, the methanol present in(Ga) is separated off from (Ga) and added to the mixture (Gii). For thepurposes of the present application, the expression “added to themixture (Gii)” covers both embodiments of the process according to theinvention in which the methanol separated off from (Ga) is firstly addedto (Gii), and the resultant mixture is fed to process step (iii), andembodiments in which the methanol separated off from (Ga) and themixture (Gii) are fed separately to process step (iii), and not untilthere do they come into contact.

The methanol may furthermore also be added to the mixture (Giii).

The present invention therefore also relates to a process as describedabove where the mixture (Ga) comprises methanol in addition to propyleneoxide, wherein methanol is separated off from (Ga) and added to themixture (Gii) or the mixture (Giii) or the mixtures (Gii) and (Giii).

The removal of the methanol from the mixture (Ga) comprising methanoland propylene oxide is preferably carried out by a distillative method,with the number of columns employed being essentially as desired.Preference is given to the use of one column. This column preferably hasat least 20, preferably at least 40 and further preferably at least 60theoretical plates.

The distillation in this column is preferably carried out at pressuresin the range from 0.3 to 10 bar, preferably from 0.4 to 2 bar andparticularly preferably from 0.6 to 1.2 bar.

The present invention therefore also relates to a process in whichmethanol is separated off from (Ga) and is as described above, whereinthe removal of the methanol is carried out in a column having at least20 theoretical plates at pressures in the range from 0.3 to 10 bar.

The reaction discharge from (i) may comprise propene which has notreacted in (i). This is preferably separated off from the reactiondischarge.

In a preferred embodiment of the process according to the invention, theremoval of the unreacted propene is carried out during the removal ofthe mixture (Gii) in process step (ii). In a further preferredembodiment, the distillative separation in (ii) is carried out by, forexample, separating off the mixture (Gii) at the bottom, separating offthe mixture (Ga) via a side take-off of the distillation column, andseparating off the unreacted propene at the top.

In a likewise preferred embodiment of the process according to theinvention, the separation in (ii) is carried out in such a way that themixture (Ga) obtained from (ii) comprises propene which has not reactedin (i) in addition to propylene oxide and possibly methanol. Thispropene is preferably separated off from (Ga) during the furtherprocess.

The present invention therefore also relates to a process as describedabove where the mixture (Ga), in addition to propylene oxide andpossibly methanol, additionally comprises propene which has not reactedin (i), wherein the propene is separated off from the mixture (Ga).

The removal of the unreacted propene from (Ga) is preferably carried outby distillation. The number of columns employed is essentially asdesired. Preference is given to the use of one column. This columngenerally has at least 5, preferably at least 10 and further preferablyat least 15 theoretical plates. The distillation in this column ispreferably carried out at pressures in the range from 0.5 to 25 bar,preferably from 0.7 to 5 bar and particularly preferably in the rangefrom 0.9 to 1.5 bar.

During the removal of the unreacted propene, the problem may undercertain circumstances occur that during removal of the propene aslow-boiling fraction, as described above, oxygen may accumulate in thislow-boiling fraction in a concentration which turns the low-boilingfraction into an ignitable mixture. This can cause a serious safety riskif propene is in turn separated off from the low-boiling fraction bydistillation, which is preferably the case if the propene is to berecycled into (i).

This problem can be solved, for example, by removing the propene fromthe low-boiler mixture by distillation and introducing an inertsubstance having a boiling point which is lower than that of propene,preferably methane, into the upper part of the separation device usedfor this purpose in such an amount that the oxygen is diluted to aconcentration at which the mixture is no longer ignitable. This processis described, for example, in EP-B 0 719 768. However, the problem ispreferably solved by using a process for the work-up of a mixturecomprising propene and oxygen in which oxygen is separated off from themixture by non-distillative methods to give a further mixture, and thepropene is separated off from the further mixture by distillation. Thisprocess is described in DE-A 100 01 401.1, which is incorporated intothe present application in its full scope in this respect by way ofreference.

In a particularly preferred embodiment of the process according to theinvention, the propene separated off is recycled into (i) as startingmaterial.

In a further particularly preferred embodiment, all said process stepsare carried out continuously. It is of course also possible to operateone or more of the steps in a batch procedure.

We claim:
 1. A process for the preparation of propylene oxide in which(i) propene is reacted with hydrogen peroxide in the presence ofmethanol to give propylene oxide, giving a mixture (Gi) comprisingpropylene oxide, methanol, water and unreacted hydrogen peroxide, (ii) amixture (Gii) comprising methanol, water and hydrogen peroxide isseparated off from the mixture (Gi), giving a mixture (Ga) comprisingpropylene oxide, and (iii) water is separated off from the mixture(Gii), giving a mixture (Giii) comprising methanol and methyl formate.2. A process as claimed in claim 1, wherein the propylene oxide isprepared using a zeolite catalyst.
 3. A process as claimed in claim 1,in which the water is separated off by distillation in (iii), wherein(w) a mixture (Gw) which principally comprises methanol and methylformate is separated off from the mixture (Gii) at the top of a firstdistillation column, (x) the mixture obtained at the bottom of the firstdistillation column is fed as feed to a second distillation column, (y)a mixture (Gy) which principally comprises methanol and methyl formateis obtained at the top of the second distillation column, and (z) themixtures (Gw) and (Gy) are combined to give the mixture (Giii).
 4. Aprocess as claimed in claim 1, wherein methanol is separated from methylformate in the mixture (Giii), and the methanol separated off isrecycled into (i).
 5. A process as claimed in claim 1, wherein thereaction in (i) is carried out in one step.
 6. A process as claimed inclaim 5, wherein the hydrogen peroxide conversion in (i) is in the rangefrom 85 to 99.99%.
 7. A process as claimed in claim 1, where the mixture(Ga) comprises methanol in addition to propylene oxide, wherein methanolis separated off from (Ga) and added to the mixture (Gii) or the mixture(Giii) or the mixtures (Gii) and (Giii).
 8. A process as claimed inclaim 7, wherein the removal of the methanol is carried out in a columnhaving at least 20 theoretical plates at pressures in the range from 0.3to 10 bar.
 9. A process as claimed in claim 1, where the mixture (Ga),in addition to propylene oxide and possibly methanol, additionallycomprises propene which has not reacted in (i), wherein the propene isseparated off from the mixture (Ga).
 10. The process as claimed in claim1, wherein the propylene oxide is prepared using a silicalite catalyst.11. The process as claimed in claim 1, wherein the propylene oxide isprepared using a titanium silicalite catalyst having the TS-1 structure.